LazyLoad (vanilla-lazyload)

LazyLoad is a fast, lightweight and flexible script that speeds up your web application by loading images only as they enter the viewport. LazyLoad supports responsive images.

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LazyLoad is a fast, lightweight and flexible script that speeds up your web application by loading your content images, videos and iframes only as they enter the viewport. It’s written in plain “vanilla” JavaScript, it leverages the IntersectionObserver API, it works with responsive images and it supports native lazy loading. See notable features for more.

➡️ Jump to: 👨‍💻 Getting started - HTML - 👩‍💻 Getting started - Script - 🥧 Recipes - 📺 Demos - 😋 Tips & tricks - 🔌 API - 😯 Notable features


👨‍💻 Getting started - HTML

In order to make your content be loaded by LazyLoad, you must use some data- attributes instead of the actual attributes. Examples below.

Lazy image:

<img alt="A lazy image" data-src="lazy.jpg">

Lazy image with low quality placeholder:

<img alt="A lazy image" src="lazy-lowQuality.jpg" data-src="lazy.jpg">

Lazy responsive image with srcset and sizes:

<img alt="A lazy image" class="lazy" 
    data-src="lazy.jpg" 
    data-srcset="lazy_400.jpg 400w, lazy_800.jpg 800w" 
    data-sizes="100w">

To have a low quality placeholder, add the src attribute pointing to a very small version of the image. E.g. src="lazy_10.jpg".

Lazy responsive image with hi-dpi support using the picture tag:

<picture>
    <source 
        media="(min-width: 1200px)" 
        data-srcset="lazy_1200.jpg 1x, lazy_2400.jpg 2x">
    <source 
        media="(min-width: 800px)" 
        data-srcset="lazy_800.jpg 1x, lazy_1600.jpg 2x">
    <img alt="A lazy image" class="lazy" 
        data-src="lazy.jpg">
</picture>

To have a low quality placeholder, add the src attribute pointing to a very small version of the image to the img tag. E.g. src="lazy_10.jpg".

Lazy responsive image with automatic WebP format selection, using the picture tag:

<picture>
    <source type="image/webp" 
        data-srcset="lazy_400.webp 400w, lazy_800.webp 800w" 
        data-sizes="100w">
    <img alt="A lazy image" class="lazy" 
        data-src="lazy.jpg" 
        data-srcset="lazy_400.jpg 400w, lazy_800.jpg 800w"
        data-sizes="100w">
</picture>

To have a low quality placeholder, add the src attribute pointing to a very small version of the image to the img tag. E.g. src="lazy_10.jpg".

Lazy background image

Single background

<div class="lazy" data-bg="url(lazy.jpg)"></div>

Multiple backgrounds

<div class="lazy" 
    data-bg="url(lazy-head.jpg), url(lazy-body.jpg), linear-gradient(#fff, #ccc)">
    ...
</div>

Notes:

Lazy video

<video class="lazy" controls width="620"
    data-src="lazy.mp4" poster="lazy.jpg">
    <source type="video/mp4" data-src="lazy.mp4">
    <source type="video/ogg" data-src="lazy.ogg">
    <source type="video/avi" data-src="lazy.avi">
</video>

Lazy iframe

<iframe class="lazy" data-src="lazyFrame.html" poster="lazy.jpg"></iframe>

👩‍💻 Getting started - Script

The latest, recommended version of LazyLoad is 12.0.0.

To polyfill or not to polyfill IntersectionObserver?

On browser NOT supporting IntersectionObserver such as Internet explorer and older versions of Safari you can choose whether or not to add a javascript polyfill for it.

If you don’t use a polyfill, LazyLoad will load all the images as soon as it’s downloaded and executed. The number of impacted users would be relatively small, so this is a completely acceptable choice.

If you prefer to load a polyfill, the regular LazyLoad behaviour is granted.

The simple, easiest way

The easiest way to use LazyLoad is to include the script from a CDN:

<script src="https://cdn.jsdelivr.net/npm/vanilla-lazyload@12.0.0/dist/lazyload.min.js"></script>

Or, with the IntersectionObserver polyfill:

<script src="https://cdn.jsdelivr.net/npm/intersection-observer@0.5.1/intersection-observer.js"></script>
<script src="https://cdn.jsdelivr.net/npm/vanilla-lazyload@12.0.0/dist/lazyload.min.js"></script>

Then, in your javascript code:

var lazyLoadInstance = new LazyLoad({
    elements_selector: ".lazy"
    // ... more custom settings?
});

To be sure that DOM for your lazy content is ready when you instantiate LazyLoad, place the script tag right before the closing </body> tag. If more DOM arrives later, e.g. via an AJAX call, you’ll need to call lazyLoadInstance.update(); to make LazyLoad check the DOM again.

if (lazyLoadInstance) {
    lazyLoadInstance.update();
}

Include via RequireJS

You can use RequireJS to dynamically and asynchronously load modules in your website.

You can also find the original W3C’S IntersectionObserver Polyfill packed in AMD so you can require it conditionally, along with LazyLoad.

Include RequireJS:

<script src="https://cdn.jsdelivr.net/npm/requirejs@2.3.6/bin/r.min.js"></script>

Then require the AMD version of LazyLoad, like this:

var lazyLoadAmdUrl = "https://cdn.jsdelivr.net/npm/vanilla-lazyload@12.0.0/dist/lazyload.amd.min.js";
var polyfillAmdUrl = "https://cdn.jsdelivr.net/npm/intersection-observer-amd@2.0.1/intersection-observer-amd.js";

/// Dynamically define the dependencies
var dependencies = [
    "IntersectionObserver" in window
        ? null // <- Doesn't require the polyfill
        : polyfillAmdUrl,
    lazyLoadAmdUrl
];

// Initialize LazyLoad inside the callback
require(dependencies, function(_, LazyLoad) {
    var lazyLoadInstance = new LazyLoad({
        elements_selector: ".lazy"
        // ... more custom settings?
    });
}

DEMO - SOURCE

Using an async script

If you prefer, it’s possible to include LazyLoad’s script using async script and initialize it as soon as it’s loaded.

To do so, you must define the options before including the script. You can pass:

<script>	
    // Set the options to make LazyLoad self-initialize	
    window.lazyLoadOptions = {	
        elements_selector: ".lazy",	
        // ... more custom settings?	
    };
</script>	

Then include the script.

<script async src="https://cdn.jsdelivr.net/npm/vanilla-lazyload@12.0.0/dist/lazyload.min.js"></script>	

Possibly place the script tag right before the closing </body> tag. If you can’t do that, LazyLoad could be executed before the browser has loaded all the DOM, and you’ll need to call its update() method to make it check the DOM again.

Using an async script + getting the instance reference

Same as above, but you must put the addEventListener code shown below before including the async script.

<script>	
    // Set the options to make LazyLoad self-initialize	
    window.lazyLoadOptions = {	
        elements_selector: ".lazy",	
        // ... more custom settings?	
    };
    // Listen to the initialization event and get the instance of LazyLoad	
    window.addEventListener('LazyLoad::Initialized', function (event) {	
        window.lazyLoadInstance = event.detail.instance;	
    }, false);	
</script>	

Then include the script.

<script async src="https://cdn.jsdelivr.net/npm/vanilla-lazyload@12.0.0/dist/lazyload.min.js"></script>	

Now you’ll be able to call its methods, like:

if (lazyLoadInstance) {
    lazyLoadInstance.update();
}

Note about Internet Explorer: because this technique uses a CustomEvent (learn more) to trigger the LazyLoad::Initialized event, you might want to add this micro polyfill to make it work on Internet Explorer.

<script>
// CustomEvent micro-polyfill for Internet Explorer
(function () {
    if (typeof window.CustomEvent === "function") {
        return false;
    }

    function CustomEvent(event, params) {
        params = params || {bubbles: false, cancelable: false, detail: undefined};
        var evt = document.createEvent("CustomEvent");
        evt.initCustomEvent (event, params.bubbles, params.cancelable, params.detail);
        return evt;
    }

    CustomEvent.prototype = window.Event.prototype;
    window.CustomEvent = CustomEvent;
})();
</script>

Local install

If you prefer to install LazyLoad locally in your project, you can!

Using npm

npm install vanilla-lazyload

Using bower

bower install vanilla-lazyload

Manual download

Download one the latest releases. The files you need are inside the dist folder. If you don’t know which one to pick, use lazyload.min.js, or read about bundles.

Local usage

Should you install LazyLoad locally, you can import it as ES module like the following:

import LazyLoad from "vanilla-lazyload";

It’s also possible (but unadvised) to use the require commonJS syntax.

More information about bundling LazyLoad with WebPack are available on this specific repo.

Usage with React

Take a look at this example of usage of React with LazyLoad on Sandbox.

This implementation takes the same props that you would normally pass to the img tag, but it renders a lazy image. Feel free to fork and improve it!

Bundles

Inside the dist folder you will find different bundles.

Filename Module Type Advantages
lazyload.min.js UMD (Universal Module Definition) Works pretty much everywhere, even in common-js contexts
lazyload.iife.min.js IIFE (Immediately Invoked Function Expression) Works as in-page <script src="...">, ~0.5kb smaller than UMD version
lazyload.amd.min.js AMD (Asynchronous Module Definition) Works with RequireJS module loader, ~0.5kb smaller than UMD version
lazyload.esm.js ES Module Exports LazyLoad so you can import it in your project both using <script type="module" src="..."> and a bundler like WebPack or Rollup

🥧 Recipes

This is the section where you can find ready to copy & paste code for your convenience.

Dynamic content

💡 Use case: when you want to lazily load images, but the number of images change in the scrolling area changes, maybe because they are added asynchronously.

HTML

The HTML to use depends on your case, see other recipes’ HTML

Javascript

var myLazyLoad = new LazyLoad();
// After your content has changed...
myLazyLoad.update();

DEMO - SOURCE - API

Scrolling panel(s)

💡 Use case: when your scrolling container is not the main browser window, but a scrolling container.

HTML

<div class="scrollingPanel" id="scrollingPanel"> 
    <!-- Set of images -->
</div>

Javascript

var myLazyLoad = new LazyLoad({
    container: document.getElementById('scrollingPanel')
});

DEMO - SOURCE - API

If you have multiple scrolling panels, you can use the following markup and code.

HTML

<div id="scrollingPanel1" class="scrollingPanel">
    <!-- Set of images -->
</div>
<div id="scrollingPanel2" class="scrollingPanel">
    <!-- Set of images -->
</div>

Javascript

var myLazyLoad1 = new LazyLoad({
    container: document.getElementById('scrollingPanel1')
});
var myLazyLoad2 = new LazyLoad({
    container: document.getElementById('scrollingPanel2')
});

DEMO - SOURCE - API

Delay loading

💡 Use case: if a your scrolls fast over your images, you might wait a short time before the images start loading. This is how.

HTML

<img class="lazy" alt="A lazy image" 
     data-src="lazy.jpg"
     width="220" height="280">

Javascript

var myLazyLoad = new LazyLoad({
    elements_selector: ".lazy",
    load_delay: 300 //adjust according to use case
});
DEMO SOURCE API

Lazy LazyLoad

💡 Use case: when you have a lot of scrolling containers in the page and you want to instantiate a LazyLoad only on the ones that are in the viewport.

HTML

<div class="horzContainer">
    <img src="" alt="Row 01, col 01" data-src="https://placeholdit.imgix.net/~text?txtsize=19&amp;txt=row_01_col_01&amp;w=200&amp;h=200">
    <img src="" alt="Row 01, col 02" data-src="https://placeholdit.imgix.net/~text?txtsize=19&amp;txt=row_01_col_02&amp;w=200&amp;h=200">
    <!-- ... -->
</div>
<div class="horzContainer">
    <img src="" alt="Row 02, col 01" data-src="https://placeholdit.imgix.net/~text?txtsize=19&amp;txt=row_02_col_01&amp;w=200&amp;h=200">
    <img src="" alt="Row 02, col 02" data-src="https://placeholdit.imgix.net/~text?txtsize=19&amp;txt=row_02_col_02&amp;w=200&amp;h=200">
    <!-- ... -->
</div>

Javascript

var lazyLoadInstances = [];
// The "lazyLazy" instance of lazyload is used (kinda improperly) 
// to check when the .horzContainer divs enter the viewport
var lazyLazy = new LazyLoad({
    elements_selector: ".horzContainer",
    // When the .horzContainer div enters the viewport...
    callback_enter: function(el) {
        // ...instantiate a new LazyLoad on it
        var oneLL = new LazyLoad({
            container: el
        });
        // Optionally push it in the lazyLoadInstances 
        // array to keep track of the instances
        lazyLoadInstances.push(oneLL);
    }
});

That’s it. Whenever a .horzContainer element enters the viewport, LazyLoad calls the callback_enter function, which creates a new instance of LazyLoad on the .horzContainer element.

DEMO - SOURCE - API


📺 Demos

Didn’t find the recipe that exactly matches your case? We have demos!

The demos folder contains 20+ use cases of LazyLoad. You might find there what you’re looking for.

Type Title Code Live demo
Content Simple lazy loaded image Code Live
Content Lazy responsive images with srcset Code Live
Content Lazy responsive images with the <picture> tag and the media attribute (art direction) Code Live
Content Lazy responsive images with srcset and sizes (using data-sizes) Code Live
Content Lazy responsive images with srcset and sizes (using plain sizes) Code Live
Content Lazy video with multiple <source> tags Code Live
Content Lazy loading background images Code Live
Content Lazy WebP images with the <picture> tag and the type attribute for WebP Code Live
Loading Asynchronous loading LazyLoad with requireJS Code Live
Loading Asynchronous loading LazyLoad + InterserctionObserver with requireJS Code Live
Loading Asynchronous loading LazyLoad with <script async> Code Live
Technique Fade in images as they load Code Live
Technique Lazily create lazyload instances Code Live
Technique How to manage the print of a page with lazy images Code Live
Technique A popup layer containing lazy images in a scrolling container Code Live
Settings Multiple scrolling containers Code Live
Settings Single scrolling container Code Live
Settings Delay loading of lazy images Code Live
Methods How to destroy() LazyLoad Code Live
Methods Adding dynamic content, then update() LazyLoad Code Live
Methods Adding dynamic content, then update() LazyLoad passing a NodeSet of elements Code Live
Methods Load punctual images using the load() method Code Live
Methods Load all images at once using loadAll() Code Live
Test Test for multiple thresholds Code Live
Test Test behaviour with hidden images Code Live
Test Test of delay loading Code Live
Test Test performance, lazy loading of hundreds of images Code Live
Native Test the native lazy loading of images WITHOUT any line of javascript, not even this script Code Live
Native Test the native lazy loading of images conditionally using the use_native option (see API) Code Live
(legacy) Conditional loading of v.8 or v.10 (no IntersectionObserver polyfill) Code Live

😋 Tips & tricks

Occupy vertical space and maintain ratio

You need to be sure that the containers of the images that are going to be lazy loaded occupy some vertical space. This because if the images have an initial height of 0, all of them will probably be inside the viewport before time, so they will be loaded all at once.

In an elastic layout where images width change, you want to keep vertical space maintaining the images height, using a width/height ratio calculation.

.image-wrapper {
    width: 100%;
    height: 0;
    padding-bottom: 66.67%; /* You define this doing height / width * 100% */
    position: relative;
}
.image {
    width: 100%;
    /*height: auto;*/
    position: absolute;
}

More info in Sizing Fluid Image Containers with a Little CSS Padding Hack by Andy Shora.

There’s also a useful SASS mixin to maintain aspect ratio on CSS tricks.

@mixin aspect-ratio($width, $height) {
  position: relative;
  &:before {
    display: block;
    content: "";
    width: 100%;
    padding-top: ($height / $width) * 100%;
  }
  > .content {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
  }
}

Show the images while they load

Images should be shown while they load, and not after, to give your users the best perceived performance. This is especially true if you use a progressive loading format like Progressive JPEG.

In order to make your images visible as soon as LazyLoad sets the src/srcset attribute to it, you can either:

Do it like that via CSS:

/* Prevents img without src to appear */
img:not([src]) {
    visibility: hidden;
}

Or instead of the above :not() selector do it using the CSS classes of class_loading and class_loaded set by LazyLoad when loading starts or is completed - see API.

Do NOT use placeholder images

We do not recommend to use a placeholder image (like a transparent pixel GIF) in your HTML.

It’s safe not to put any value in the src nor srcset attributes, even if your HTML won’t validate by a static code analyzer. The reason is that once JavaScript is executed, those values will be set by LazyLoad. For SEO, if the client is a crawler like Googlebot, it will be detected by LazyLoad which will fix the HTML.

Dealing with Microsoft Edge problems

According to what reported in #152, for Microsoft Edge to fire the IntersectionObserver for an img element, it must have a size. Since imgs are displayed inline-block as standard, MS Edge (version not specified) doesn’t read them correctly.

By setting the following, Edge is able to see the images and they get loaded.

img[data-src],
img[data-srcset] {
  display: block;
  min-height: 1px;
}

🔌 API

Constructor arguments

The new LazyLoad() instruction you execute on your page can take two parameters:

Parameter What to pass Required Default value Type
Options The option object for this instance of LazyLoad No {} Plain Object
Nodeset A NodeSet of elements to execute LazyLoad on No null NodeSet

The most common usage of LazyLoad constructor is to pass only the options object (see “options” in the next section). For example:

var aLazyLoad = new LazyLoad({ 
    /* options here */ 
});

In the rare cases where you can’t or don’t want to select the elements using elements_selector and you have a reference variable to your elements set (can be a NodeSet or an array of elements), you can pass the elements set as the second parameter.

var elementsToLazyLoad = getElementSetFromSomewhere();
var aLazyLoad = new LazyLoad({ 
    /* options here */ 
}, elementsToLazyLoad);

Options

For every instance of LazyLoad you can pass in some options, to alter its default behaviour. Here’s the list of the options.

Name Meaning Default value Example value
container The container of the elements in the elements_selector option. document document.querySelector('.scrollPanel')
elements_selector The CSS selector of the elements to load lazily, which will be selected as descendants of the container object. "img" ".images img.lazy"
threshold A number of pixels representing the outer distance off the scrolling area from which to start loading the elements. 300 0
thresholds Similar to threshold, but accepting multiple values and both px and % units. It maps directly to the rootMargin property of IntersectionObserver (read more), so it must be a string with a syntax similar to the CSS margin property. You can use it when you need to have different thresholds for the scrolling area. It overrides threshold when passed. null "500px 10%"
data_src The name of the data attribute containing the original image source, excluding the "data-" part. E.g. if your data attribute is named "data-src", just pass "src" "src" "original"
data_srcset The name of the data attribute containing the original image source set in either img and source tags, excluding the "data-" part. E.g. if your data attribute is named "data-srcset", just pass "srcset" "srcset" "original-set"
data_sizes The name of the data attribute containing the sizes attribute to use, excluding the "data-" part. E.g. if your data attribute is named "data-sizes", just pass "sizes" "sizes" null
data_bg The name of the data attribute containing the value of background-image to load lazily, excluding the "data-" part. E.g. if your data attribute is named "data-bg", just pass "bg". The attribute value must be a valid value for background-image, including the url() part of the CSS instruction. "bg" "url(img1.jpg), url(img2.jpg)"
class_loading The class applied to the elements while the loading is in progress. "loading" "lazy-loading"
class_loaded The class applied to the elements when the loading is complete. "loaded" "lazy-loaded"
class_error The class applied to the elements when the element causes an error. "error" "lazy-error"
load_delay The time (in milliseconds) each image needs to stay inside the viewport before its loading begins. 0 300
auto_unobserve A boolean that defines whether or not to automatically unobserve elements that was already revealed true false
callback_enter A callback function which is called whenever an element enters the viewport. null (el)=>{console.log("Entered", el)}
callback_exit A callback function which is called whenever an element exits the viewport. null (el)=>{console.log("Exited", el)}
callback_reveal A callback function which is called whenever an element starts loading. null (el)=>{console.log("Loading", el)}
callback_set Deprecated from version 11 → It still works, but please update your code to use callback_reveal instead. null (el)=>{console.log("Loading", el)}
callback_loaded A callback function which is called whenever an element finishes loading. null (el)=>{console.log("Loaded", el)}
callback_error A callback function which is called whenever an element triggers an error. null (el)=>{console.log("Error", el)}
callback_finish A callback function which is called when there are no more elements to load and all elements have been downloaded. null ()=>{console.log("Finish")}
use_native This boolean sets whether or not to use native lazy loading. On browsers that supports it, LazyLoad will set the loading="lazy" attribute on images and iframes, and delegate their loading to the browser. false true

Methods

You can call the following public methods on any instance of LazyLoad.

Method name Effect
update() Make LazyLoad to check for new lazy images in the container, using the elements_selector option.
loadAll() Loads all the lazy images right away, no matter if they are inside or outside the viewport.
load(element, force) Immediately loads any lazy element, even if it isn’t selectable by the elements_selector option. Note that this method works only once on a specific element, unless you force it passing true as the second parameter.
destroy() Destroys the instance, unsetting instance variables and removing listeners.

😯 Notable features

It works with your favourite library or framework

As LazyLoad doesn’t rely on jQuery, you can use it in web applications using Angular, React or Vue.js without the need to include jQuery.

Intersection Observer API for optimized CPU usage

Instead of listening to the scroll and resize events, LazyLoad uses the Intersection Observer API which is a new, blazing fast method to detect if an element is inside the browser viewport. Your users will see the difference in slow and even in fast devices or computers.

Support for responsive images

LazyLoad supports responsive images, both via the srcset & sizes attributes and via the picture tag.

SEO friendly

LazyLoad doesn’t hide your images from search engines, even if you don’t specify any initial src for your image.

Tested on real browsers

Legacy browsers support is from IE 9 up. This script is tested in every browser before every release using BrowserStack live, thanks to the BrowserStack Open Source initiative.

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